Current infrared detector systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connectable to sophisticated processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and monitor sources of infrared radiation. A contemporary subarray of detectors may contain 256 detectors on a side, or a total of 65,536 detectors. The size of each square detector may be approximately 0.009 cm on a side with 0.00127 cm spacing between detectors. Such a subarray would therefore be 2.601 cm on a side. The subarray may, in turn, be joined to form an array that connects many millions of detectors. As might be expected, the processing requirements for such a highly populated and closely spaced array are strenuous, particularly considering the demands of a space environment.
The response of the infrared detector elements and their associated electronic processing channel is preferably linear with an output in volts per watt of absorbed infrared radiation. A DC offset is associated with each detector and signal processing channel and is defined as fixed pattern noise. The detector response and DC offset are unique for each channel, i.e. they are different for each channel but all within a given range of values. Satisfactory detector input circuits exist to reduce or eliminate noise while providing near unity gain and stable operation. One such circuit is disclosed in U.S. Pat. No. 4,633,086 for INPUT CIRCUIT FOR INFRARED DETECTOR, issued to William J. Parrish, and assigned to the common assignee hereof.
A further requirement particularly significant in highly populated infrared detection systems, concerns the circuit for normalizing gain among the analog signal channels. Gain normalization to a high level is required by the sophisticated algorithms which analyze the detector array outputs. The variation in gain from detector channel to another increases the dynamic range requirements for the output amplifier when detector channels are multiplexed into a common output amplifier. The dynamic range of the analog-to-digital converter and the signal processing hardware must likewise have sufficient dynamic range to handle the multiplexed data. Increasing the dynamic range requirements of the output amplifier, analog-to-digital converter, and signal processing hardware results in an increase in system weight and cost. Therefore, it is desirable to maintain the best possible uniformity in gain among all detector channels. One suitable gain normalization circuit is disclosed in U.S. Pat. No. 5,039,879 for GAIN NORMALIZATION CIRCUIT, issued to William J. Parrish, and assigned to the common assignee hereof.
Having addressed the concern of noise and gain normalization, the signal processing circuitry must further have a sufficient range of response to accurately identify and quantify inputs from both dim targets and bright targets. For example, the infrared detection system must be sufficiently sensitive to distinguish and quantify dim infrared images corresponding to particular events of interest. However, the circuit must also produce an accurate response, i.e. avoid saturation, when a bright target is detected. The present invention addresses these concerns by providing a dual integration circuit which utilizes a long integration period to detect and quantify dim targets, as well as a short integration period for detecting and quantifying bright targets. In view of the constraints of highly populated, space based systems, the present invention implements a dual integration circuit in each channel of the infrared detection system, on the same chip as the input and gain normalization circuitry.